1. Technical Field
The present invention generally relates to a method for evaluating asphaltene content, stability and solubility characteristics of a hydrocarbon-containing feedstock employing a low volume, in-line filtration device.
2. Description of the Related Art
Hydrocarbon materials, such as heavy oils, petroleum residua, coal tars, shale oils, asphalts, or the like can comprise polar core materials, such as asphaltenes, dispersed in lower polarity solvent(s). Intermediate polarity material(s), usually referred to as resin(s), can associate with the polar core materials to maintain a homogeneous mixture of the components.
Refinery processes, including but not limited to, atmospheric or vacuum distillation, visbreaking, hydrocracking, delayed coking, Fluid Coking, Flexicoking, hydrotreatment, delay coker or Eureka process that convert hydrocarbon materials to lighter distillate fuels that require heating for distillation, hydrogen addition, or carbon rejection (coking). However, when using conventional refinery processes, the efficiency of converting such hydrocarbon material may be limited by transition of the hydrocarbon material of homogeneous mixture to a hydrocarbon material of heterogeneous mixture. The transition to the heterogeneous mixture may include the formation of insoluble carbon-rich deposits, including the formation of coke or coke-containing materials. As such, any reduction in carbon deposition, or increase in the distillation yield during the thermal processing of hydrocarbon material can have a significant impact on the manner or economics of hydrocarbon processing.
Asphaltenes are organic heterocyclic macromolecules which occur in crude oils. Under normal reservoir conditions, asphaltenes are usually stabilized in the crude oil by maltenes and resins that are chemically compatible with asphaltenes, but that have lower molecular weight. Polar regions of the maltenes and resins surround the asphaltene while non-polar regions are attracted to the oil phase. Thus, these molecules act as surfactants and result in stabilizing the asphaltenes in the crude. However, changes in pressure, temperature or concentration of the crude oil can alter the stability of the dispersion and increase the tendency of the asphaltenes to agglomerate into larger particles. As these asphaltene agglomerates grow, so does their tendency to precipitate out of solution.
One of the problems encountered in crude oil production and refining is asphaltene precipitation. Generally, unwanted asphaltene precipitation is a concern to the petroleum industry due to, for example, plugging of an oil well or pipeline as well as stopping or decreasing oil production. Also, in downstream applications, asphaltenes are believed to be the source of coke during thermal upgrading processes thereby reducing and limiting yield of residue conversion. In catalytic upgrading processes, asphaltenes can contribute to catalyst poisoning by coke and metal deposition thereby limiting the activity of the catalyst.
Asphaltenes can also cause fouling in, for example, heat exchangers and other equipment in a refinery. Fouling in heat transfer equipment used for streams of petroleum origin can result from a number of mechanisms including chemical reactions, corrosion and the deposit of materials made insoluble by the temperature difference between the fluid and heat exchange wall. The presence of insoluble contaminants may exacerbate the problem: blends of a low-sulfur, low asphaltene (LSLA) crude oil and a high-sulfur, high asphaltene (HSHA) crude, for example, may be subject to a significant increase in fouling in the presence of iron oxide (rust) particulates. Subsequent exposure of the precipitated asphaltenes over time to the high temperatures then causes formation of coke as a result of thermal degradation.
Equipment fouling is costly to petroleum refineries and other plants in terms of lost efficiencies, lost throughput, and additional energy consumption, and, with the increased cost of energy, heat exchanger fouling has a greater impact on process profitability. Higher operating costs also accrue from the cleaning required to remove fouling. While many types of refinery equipment are affected by fouling, cost estimates have shown that the majority of profit losses occur due to the fouling of whole crude oils, blends and fractions in pre-heat train exchangers.
Fouling is generally defined as the accumulation of unwanted materials on the surfaces of processing equipment. In petroleum processing, fouling is the accumulation of unwanted hydrocarbon-based deposits on, for example, heat exchanger surfaces. It has been recognized as a nearly universal problem in design and operation of refining and petrochemical processing systems, and affects the operation of equipment in two ways. First, the fouling layer has a low thermal conductivity. This increases the resistance to heat transfer and reduces the effectiveness of the heat exchangers. Second, as deposition occurs, the cross-sectional area is reduced, which causes an increase in pressure drop across the apparatus and creates inefficient pressure and flow in the heat exchanger.
One of the more common causes of rapid fouling, in particular, is the formation of coke that occurs when crude oil asphaltenes are overexposed to heater tube surface temperatures. The liquids on the other side of the exchanger are much hotter than the whole crude oils and result in relatively high surface or skin temperatures. Certain asphaltenes can precipitate from the oil and adhere to these hot surfaces. Another common cause of rapid fouling is attributed to the presence of salts and particulates. Salts/particulates can precipitate from the crude oils and adhere to the hot surfaces of the heat exchanger. Inorganic contaminants play both an initiating and promoting role in the fouling of whole crude oils and blends. Iron oxide, iron sulfide, calcium carbonate, silica, sodium and calcium chlorides have all been found to be attached directly to the surface of fouled heater rods and throughout the coke deposit.
The cleaning process, whether chemical or mechanical, in petroleum refineries and petrochemical plants often causes costly shutdowns. A majority of refineries practice off-line cleaning of heat exchanger tube bundles based on scheduled time or usage or on actual monitored fouling conditions. Reduction in the extent of fouling will lead to increased run lengths, improved performance and energy efficiency while also reducing the need for costly fouling mitigation options.
In addition, oil refining gives rise to dark heavy high-boiling oil fractions and their mixtures, of which bitumen and heavy fuel oil are made, among other things. The use and storability of these oil raffinates are impaired by the poor solubility or precipitation of asphaltenes in the oil. Thus, susceptibility of the asphaltene components to precipitate determines the stability or storability of the oil, and this depends both on the oil production process used and on the raw materials.
Present methodologies use a vessel connected to a high performance liquid chromatography (HLPC), also known as high pressure liquid chromatography, to evaluate asphaltene precipitation. For example, one such method disclosed in U.S. Patent Application Publication No. 2011020950 involves in-vessel precipitation of asphaltenes using a vessel consisting of an inert non-porous column. These methods have proven to be faster and required less solvent amounts than traditional technologies. However, there are problems associated with the use of a column packed with, for example, Teflon. One such problem is that the filling of a column using, for example, the “tap-fill” method for packing of rigid solids, produces columns that will have different performances because of the difficulties in forming an optimally packed column bed. This, in turn, leads to poorer reproducibility and repeatability of the methodology. In fact, a column prepared by different trained personnel as well as a column prepared by the same personnel produce results with differences of more than 10%.
Another problem is the degradation of the column due to asphaltene adsorption. In addition, the use of a packed column also results in the formation of preferential channels which requires its frequent replacement for a new fresh-packed column. This highly affects repeatability of the methodology because of poorer column-to-column reproducibility. In particular, for processed or paraffinic containing samples, the degradation of the column can be very fast (e.g., less than a month). This, in turn, requires the time-consuming task of preparing a new column which increases capital and operational costs.
Yet another problem is that the use of a column produces very broad peaks when analyzing the solubility profile of the eluted fractions with a liquid chromatography detector. An unexpected result of the present invention is that the use of a filter instead of a column improves significantly the sharpness of the peaks. This is believed to be due to the comparatively lower volume of the filter. For example, a packed column produced by the “tap-fill” method has a high volume which leads to very broad peaks. This reduces the sensitivity and the repeatability of the method due to small signal/noise ratio. In order to keep the peaks as narrow as possible, large flow rates are required when the column is used. This, in turn, limits the type of detectors that can be used with the concomitant reduction in sensitivity. Moreover, even flow rates of the liquid sample as large as 4 mL/min produce peaks that are broader than those obtained by conventional liquid chromatography using much lower flow rates. Also, large flow rates increase solvent consumption.
Finally, another problem is that high volume columns also increase the pressure differential across the HPLC lines, shortening maintenance cycles and lowering life times for pumps and seal pumps.
Another area for improvement arises from the unexpected discovery that the precipitation of the asphaltenes outside the vessel helps in the production of sharper peaks. By using a large sample/precipitant solvent ratio (around 1/10,000), the precipitation occurs instantaneously when the sample enters into contact with the solvent thereby producing a narrow band of asphaltenes, while precipitation in-vessel induces a broader distribution of the asphaltenes within the vessel that also leads to broader peaks. It is undesirable to have broad peaks as they are detrimental for repeatability and limit of detection.
Another important factor in the development of better methodologies to evaluate asphaltene stability is the proper selection of the solvents. Previously, U.S. Pat. No. 8,492,154 (“the '154 patent) disclosed that polarity can be used to select the solvents. It was stated in the '154 patent that the larger the polarity of the solvent the larger the solubility of asphaltenes in the solvent. However, the characterization of a solvent by means of its “polarity” is an unsolved problem as indicated by Riechardt, C., “Solvents and Solvent Effects in Organic Chemistry” Wiley-Vch Verlag GmbH & Co., Weinheim, Germany, 2004, p. 68 (“Riechardt”). In fact, Riechardt points out that the term “polarity” itself has not been precisely defined. According to Riechardt, polarity might be interpreted as: (a) the permanent dipole moment of a compound, (b) its relative permittivity and (c) the sum of all those molecular properties responsible for all the interaction forces between solvent and solute properties.
As examples, Tables 1 and 2 below show two different polarity scales and how they are unable to determine appropriate asphaltene solvents. First, Table 1 shows a polarity scale published by Barton, Allan F. M. “Handbook of Solubility” CRC Press, 2nd Edition, 1991, p. 88-93 (see, Tables 7-9), In this scale, polarity is defined by the polar term in the solubility parameter.
Second, Table 2 shows a second so called “polarity” scale defined by Rutan et al., L. R. J. Chromatogr., 463, 21, 1989. This scale cannot be used to select the solvents for asphaltene solubilization. As seen in Tables 1 and 2, “higher polarity” of a solvent does not correlate with higher solubility of asphaltenes in the solvent.
TABLE 1SolventPolarityObservationReferenceHeptane0.0Does not dissolveMitchell, et al.,asphaltenesFuel, 1973, 52, 151.Cyclohexane0.0Dissolves asphaltenesMitchell, et al.,Fuel, 1973, 52, 151.Benzene0.0Dissolves asphaltenesMitchell, et al.,Fuel, 1973, 52, 151.Toluene1.4Dissolves asphaltenesMitchell, et al.,Fuel, 1973, 52, 151.Diethylether4.6Does not dissolveAl-Jarrah et al., FuelasphaltenesSci. Technol. Int.1986,4, 249.Tetrahydrofuran7.6Dissolves asphaltenesCeballo, et al.,Petroleum Scienceand Technology,1999, 17, 783.Pyridine7.6Dissolves asphaltenesMitchell, et al.,Fuel, 1973, 52, 151.Ethyl Acetate10.6Does not dissolveCarbognani, et al.,asphaltenesEnergy Fuels, 2002,16, 1348.Acetone12.9Does not dissolveCarbognani, et al.,asphaltenesEnergy Fuels, 2002,16, 1348.Acetonitrile18.4Does not dissolveCarbognani, et al.,asphaltenesE. Energy Fuels,2002, 16, 1348.
TABLE 2SolventPolarity ObservationReferenceHexane−0.14Does not dissolveMitchell, et al.,asphaltenesFuel, 1973, 52, 151.Cyclohexane0.17Partially dissolvesMitchell, et al.,asphaltenesFuel, 1973, 52, 151.Toluene2.68Dissolves asphaltenesMitchell, et al.,Fuel, 1973, 52, 151.Diethylether3.15Does not dissolveAl-Jarrah et al., FuelasphaltenesSci. Technol. Int.1986,4, 249.Ethyl Acetate4.24Does not dissolveCarbognani, et al.,asphaltenesE. Energy Fuels,2002, 16, 1348.Tetrahydrofuran4.28Dissolves asphaltenesCeballo, et al.,Petroleum Scienceand Technology,1999, 17, 783.Methylene4.29Dissolves asphaltenesMitchell, et al.,ChlorideFuel, 1973, 52, 151.Pyridine5.53Dissolves asphaltenesMitchell, et al.,Fuel, 1973, 52, 151.Acetonitrile5.64Does not dissolveCarbognani, et al.,asphaltenesEnergy Fuels, 2002,16, 1348.
In the same manner, the use of solvent power or solvent strength without defining a scale to determine the best solvents for asphaltene dissolution is misleading. Solvent power or solvent strength is also ambiguous since its quantification requires a scale. For example, there are scales that apply exclusively to hydrocarbons (see, e.g., Barton, Allan F. M. “Handbook of Solubility” CRC Press, 2nd Edition, 1991, p. 288-289.) and cannot be used for oxygenated agents (e.g., alcohols, ketones, etc.). It is clear that without a scale definition, polarity and solvent power are ill-defined concepts.
Accordingly, it is clear that there is a need to define a proper scale for the successful selection of solvents. The '154 patent uses solubility parameter that comprises three components: dispersion, polar and hydrogen bonding. Each of the components related to a specific type of intermolecular interactions. The '154 patent states that this scale can be used to select which solvents are best to dissolve asphaltenes. However, a larger solubility parameter does not correlate with better solvency of asphaltenes into the solvent as shown below in Table 3.
TABLE 3δTSolvent(MPa0.5)ObservationReferenceHeptane15.3Does not dissolveMitchell, et al.,asphaltenesFuel, 1973, 52, 151.Diethyl ether15.8Does not dissolveAl-Jarrah et al.,asphaltenesFuel Sci. Technol.Int. 1986,4, 249.Cyclohexane16.8Dissolves asphaltenesMitchell, et al.,Fuel, 1973, 52, 151.Ethyl Acetate18.1Does not dissolveCarbognani, et al.,asphaltenesEnergy Fuels, 2002,16, 1348.Toluene18.2Dissolves asphaltenesMitchell, et al.,Fuel, 1973, 52, 151.Benzene18.6Dissolves asphaltenesMitchell, et al.,Fuel, 1973, 52, 151.Tetrahydrofuran19.4Dissolves asphaltenesCeballo, C et al.,Petroleum Scienceand Technology,1999, 17, 783.Acetone20.0Does not dissolveCarbognani, et al.,asphaltenesEnergy Fuels, 2002,16, 1348.Methylene20.3Dissolves asphaltenesMitchell, et al.,chlorideFuel, 1973, 52, 151.Pyridine21.8Dissolves asphaltenesMitchell, et al.,Fuel, 1973, 52, 151.Acetonitrile24.4Does not dissolveCarbognani, et al.,asphaltenesEnergy Fuels, 2002,16, 1348.Water48.0Does not dissolveMoschopedis, et al..asphaltenesFuel, 1971, 50, 34
An improve method to select the solvents and its order to determine solubility characteristics of the asphaltenes can be based on the dispersion component of the solubility parameter. This component of the solubility parameter takes into account the forces related to the polarizability of the molecules and is commonly associated with their size and shape and it is the predominant interaction force among asphaltenes. The other two components of the solubility parameter, i.e., polar and hydrogen bonding, are minor contributors to asphaltene interactions. This is demonstrated in Table 1 above, where the polarity scale represented by these two contributions cannot be used to select the solvents. In contrast, the dispersion component of the Hansen Parameters (See Barton, A. F. M.; Handbook of Solubility Parameters and Other Cohesion Parameters, Second Edition CRC Pess, USA, 1991, p 104-107) as shown below in Table 4 can be used to select the solvents as it shows the right order in terms of solubility of asphaltenes
TABLE 4δdSolvent(MPa0.5)ObservationReferenceWater12.3Does not dissolveMoschopedis, et al.,asphaltenesFuel, 1971, 50, 34Diethyl ether14.5Does not dissolveAl-Jarrah et al.,asphaltenesFuel Sci. Technol.Int. 1986,4, 249.Methanol15.1Does not dissolveCarbognani, et al.,asphaltenesEnergy Fuels, 2002,16, 1348.Heptane15.3Does not dissolveMitchell, et al.,asphaltenesFuel, 1973, 52, 151.Acetonitrile15.3Does not dissolveCarbognani, et al.,asphaltenesEnergy Fuels, 2002,16, 1348.Acetone15.5Does not dissolveCarbognani, et al.,asphaltenesEnergy Fuels, 2002,16, 1348.Ethyl Acetate15.8Does not dissolveCarbognani, et al.,asphaltenesEnergy Fuels, 2002,16, 1348.Cyclohexane16.8Dissolves asphaltenesMitchell, D et al.,Fuel, 1973, 52, 151.Tetrahydrofuran16.8Dissolves asphaltenesCeballo, et al.,Petroleum Scienceand Technology,1999, 17, 783.Toluene18.2Dissolves asphaltenesMitchell, et al.,Fuel, 1973, 52, 151.Methylene18.2Dissolves asphaltenesMitchell, et al.,chlorideFuel, 1973, 52, 151.Benzene18.4Dissolves asphaltenesMitchell, et al.,Fuel, 1973, 52, 151.Pyridine19.0Dissolves asphaltenesMitchell, et al.,Fuel, 1973, 52, 151.
It would be therefore desirable to provide improved methods for determining, for example, asphaltene content and asphaltene stability, in a hydrocarbon-containing material that can be carried out in a simple, cost efficient and repeatable manner.